During a typical influenza season, influenza A viruses primarily cause mortality in the very young and old;conversely, pandemic influenza viruses, including the 1918 H1N1 subtype and avian H5N1 viruses, cause high rates of mortality in young adults (ages 18-40) in addition to children and the elderly. During the 1918 epidemic, death rates were significantly higher among men than women. Conversely, in 2006, human cases of H5N1 avian influenza were significantly higher among young women than men. Whether gender differences in mortality from influenza virus infection during pandemics are due to dimorphic exposure or susceptibility to infection has not been resolved. To date, there are no reports of systematic examination of gender differences in response to influenza A viruses in either humans or animal models. The overarching aim of this proposal is to develop an animal model that enables us to test the hypothesis that gender differences in susceptibility to influenza virus infection reflect differential regulation of inflammation and development of immunopathology. Preliminary data from my laboratory reveal that following intranasal inoculation with the virulent mouse-adapted influenza A virus A/PR/8/34 (PR8;H1N1), morbidity and mortality are higher in female than male C57BL/6 mice. Our data further indicate that this dimorphism is dependent on circulating sex steroid hormones. We hypothesize that induction of excessively high proinflammatory responses to influenza virus infection may lead to immunopathology and death in females. Specific Aim 1 will test the hypothesis that gender differences in the pathogenicity of influenza A viruses are associated with sex steroid hormones, production of pro- and anti- inflammatory cytokines/chemokines, and signaling through the Myd88 pathway. Specific Aim 2 will establish whether the balance between proinflammatory and anti-inflammatory T cell responses during infection, specifically responses of regulatory T cells and IL-17-producing T cells, differs between the sexes and is altered by sex steroids during influenza A virus infection. The studies proposed will provide a comprehensive examination of innate and adaptive immunological factors mediating gender differences in inflammatory responses to influenza which will serve as an important consideration for pandemic preparedness. Development of the proposed animal model for systematic examination of gender-specific responses to influenza also will translate into testable hypotheses for the determination of gender differences in responses to influenza A virus infection and vaccination in humans. PUBLIC HEALTH RELEVANCE: The burden of influenza as a global infectious disease is well recognized;missing from current initiatives is an understanding of the causes of gender differences in susceptibility to infection. Preliminary data from my laboratory reveal that following infection with virulent influenza A virus, morbidity and mortality are higher in female than male mice and we hypothesize that induction of excessively high proinflammatory responses to influenza virus may lead to immunopathology and death in females. The studies proposed will provide essential information about immune regulation in males and females, which may impact our understanding of infection as well as vaccine efficacy in different populations-a critical consideration for pandemic preparedness.